In many innovative transmission methods in the field of communications technology, the trend in technology has been towards the development of ever broader transmission bandwidths and ever higher bandwidth efficiencies. As a result of the sustained and steady further development of electronic components, it has become possible to generate signals in modulators with broader bandwidth and higher quality, and at the same time, to receive with a corresponding quality in affordably manufactured receivers (set-top boxes, modems, mobile telephones). However, with the same level of technological development, the performance of the transmitter components is often in advance of the performance of the corresponding receiver components, for example, DACs versus ADCs. Furthermore, additional performance deficits occur in the receiver as a result of estimation errors in the synchronisation mechanisms. Nevertheless, this performance difference is not particularly restricting for transmitters and receivers participating in the actual communications infrastructure because the transmission channel generally degrades the signal and, accordingly, the receiver performance, for example, with regard to intrinsic MER (MER=modulation-error ratio) need not have the same value as in the transmitter.
By contrast, this difference is problematic for a measurement receiver with which the performance of the transmitter must be demonstrated directly at its output, because the intrinsic receiver MER should be as much as 6-10 dB higher than the value to be demonstrated, for example, in order to demonstrate a modulator MER.
As one example of this problem, the specification for DOCSIS 3.1 OFDM PHY will be used in the following. DOCSIS 3.1 is a bidirectional transmission standard for analog cable networks. In the downstream, OFDM channels are used with a bandwidth of up to 192 MHz and a sub-carrier spacing of 25 kHz or 50 kHz. At the same time, the desired maximal modulation order for each sub-carrier is 4096 QAM or respectively, in the medium-term-future, as much as 16384 QAM. To achieve these modulation orders, the specification for the downstream modulator requires an MER greater than 50 dB as an average of all sub-carriers within a 192 MHz channel.
A measurement receiver would therefore have to achieve an MER of at least 56 dB over the entire frequency range with a reception bandwidth of 192 MHz. This requirement would make the measurement receiver either extremely effort-intensive or even incapable of realisation. Using current technology, this MER performance could be realised with a reasonable effort, for example, with a reception bandwidth of 10 MHz. In general, however, the DOCSIS 3.1 specification does not provide for it to be able to synchronise with a receiver on only a part of the sub-carrier and therefore to implement a narrowband MER measurement.
For example, the German patent application DE 10 2006 057 316 A1 shows a measurement receiver for the reception of OFDM signals. However, the latter operates with a broad bandwidth in a single step and can therefore only achieve an accuracy of measurement which, with an identical technology level, is disposed below an accuracy of a transmitter to be measured.